METHOD FOR RANGING DEVICES USING CODE SEQUENCES IN WLANs

ABSTRACT

Methods and apparatus for using code-division multiple access (CDMA) to transmit information via orthogonal frequency-division multiplexing (OFDM) to convey information from user terminals to an access point (AP) in a wireless local area network (WLAN) are provided. By using CDMA to convey information, a propagation delay between an access point (AP) and a user terminal may be determined by the AP, and timing adjustment information based on the delay may be sent to the user terminal. In this manner, subsequent uplink (UL) transmissions from multiple user terminals may be received simultaneously by the AP, despite the multiple user terminals having potentially different propagation delays.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/090,515 filed Aug. 20, 2008, which is herein incorporated byreference in its entirety.

1. Field

Embodiments of the present disclosure generally relate to wirelesscommunications and, more particularly, to using code-division multipleaccess (CDMA) to transmit information via orthogonal frequency-divisionmultiplexing (OFDM) in wireless local area networks (WLANs).

2. Background

In order to address the issue of increasing bandwidth requirementsdemanded for wireless communication systems, different schemes are beingdeveloped to allow multiple user terminals to communicate with a singlebase station by sharing the same channel (same time and frequencyresources) while achieving high data throughputs. Spatial DivisionMultiple Access (SDMA) represents one such approach that has recentlyemerged as a popular technique for the next generation communicationsystems. SDMA techniques may be adopted in several emerging wirelesscommunications standards such as IEEE 802.11 (IEEE is the acronym forthe Institute of Electrical and Electronic Engineers, 3 Park Avenue,17th floor, New York, N.Y.) and Long Term Evolution (LTE).

In SDMA systems, a base station may transmit or receive differentsignals to or from a plurality of mobile user terminals at the same timeand using the same frequency. In order to achieve reliable datacommunication, user terminals may need to be located in sufficientlydifferent directions. Independent signals may be simultaneouslytransmitted from each of multiple space-separated antennas at the basestation. Consequently, the combined transmissions may be directional,i.e., the signal that is dedicated for each user terminal may berelatively strong in the direction of that particular user terminal andsufficiently weak in directions of other user terminals. Similarly, thebase station may simultaneously receive on the same frequency thecombined signals from multiple user terminals through each of multipleantennas separated in space, and the combined received signals from themultiple antennas may be split into independent signals transmitted fromeach user terminal by applying the appropriate signal processingtechnique.

A multiple-input multiple-output (MIMO) wireless system employs a number(N_(T)) of transmit antennas and a number (N_(R)) of receive antennasfor data transmission. A MIMO channel formed by the N_(T) transmit andN_(R) receive antennas may be decomposed into N_(S) spatial channels,where, for all practical purposes, N_(S)≦min{N_(T),N_(R)}. The N_(S)spatial channels may be used to transmit N_(S) independent data streamsto achieve greater overall throughput.

In a multiple-access MIMO system based on SDMA, an access point cancommunicate with one or more user terminals at any given moment. If theaccess point communicates with a single user terminal, then the N_(T)transmit antennas are associated with one transmitting entity (eitherthe access point or the user terminal), and the N_(R) receive antennasare associated with one receiving entity (either the user terminal orthe access point). The access point can also communicate with multipleuser terminals simultaneously via SDMA. For SDMA, the access pointutilizes multiple antennas for data transmission and reception, and eachof the user terminals typically utilizes less than the number of accesspoint antennas for data transmission and reception. When SDMA istransmitted from an access point, N_(S)≦min{N_(T), sum(N_(R))}, wheresum(N_(R)) represents the summation of all user terminal receiveantennas. When SDMA is transmitted to an access point,N_(S)≦min{sum(N_(T)), N_(R)}, where sum(N_(T)) represents the summationof all user terminal transmit antennas.

SUMMARY

Certain embodiments of the present disclosure provide a method forranging a device in a wireless communications system. The methodgenerally includes transmitting a plurality of orthogonalfrequency-division multiplexing (OFDM) symbols representing a codesequence in an OFDM frame, receiving timing information from an accesspoint (AP), the timing information based on when the plurality of OFDMsymbols was received by the AP, and using the timing information tocontrol a start time of a subsequent uplink (UL) transmission.

Certain embodiments of the present disclosure provide a computer-programproduct for ranging a device in a wireless communications system. Thecomputer-program product typically includes a computer-readable mediumhaving instructions stored thereon, the instructions being executable byone or more processors. The instructions generally include instructionsfor transmitting a plurality of OFDM symbols representing a codesequence in an OFDM frame, instructions for receiving timing informationfrom an AP, the timing information based on when the plurality of OFDMsymbols was received by the AP, and instructions for using the timinginformation to control a start time of a subsequent UL transmission.

Certain embodiments of the present disclosure provide an apparatus forwireless communications. The apparatus generally includes means fortransmitting a plurality of OFDM symbols representing a code sequence inan OFDM frame, means for receiving timing information from an AP, thetiming information based on when the plurality of OFDM symbols wasreceived by the AP, and means for using the timing information tocontrol a start time of a subsequent UL transmission.

Certain embodiments of the present disclosure provide a mobile device.The mobile device generally includes a transmitter configured totransmit a plurality of OFDM symbols representing a code sequence in anOFDM frame, a receiver configured to receive timing information from anAP, the timing information based on when the plurality of OFDM symbolswas received by the AP, and logic for using the timing information tocontrol a start time of a subsequent UL transmission.

Certain embodiments of the present disclosure provide a wirelesscommunications system. The system generally includes a user terminalconfigured to transmit a plurality of OFDM symbols representing a codesequence in an OFDM frame; and an AP configured to receive the pluralityof symbols in the OFDM frame, perform correlation to detect the codesequence from the plurality of OFDM symbols, determine timinginformation based on the correlation and when the plurality of OFDMsymbols was received by the AP, and transmit the timing information suchthat the user terminal may use the timing information to control a starttime of a subsequent UL transmission.

Certain embodiments of the present disclosure provide a wirelesscommunications system. The system generally includes a plurality of userterminals, wherein each user terminal is configured to transmit aplurality of OFDM symbols representing a code sequence in an OFDM framesuch that a plurality of OFDM frames are simultaneously transmitted bythe plurality of user terminals; and an AP configured to receive theplurality of OFDM frames and, for each received OFDM frame in theplurality of OFDM frames, configured to perform correlation to detectthe code sequence from the plurality of OFDM symbols in the OFDM frame,determine timing information based on the correlation and on when theplurality of OFDM frames was received by the AP, and transmit the timinginformation such that the plurality of user terminals may use the timinginformation for each of the plurality of user terminals to control starttimes of subsequent UL transmissions to be simultaneously received bythe AP.

Certain embodiments of the present disclosure provide a method forranging multiple devices in a wireless communications system. The methodgenerally includes transmitting a plurality of OFDM symbols in an OFDMframe, receiving timing information from an AP, the timing informationbased on when the plurality of OFDM symbols was received by the AP, andusing the timing information to control a start time of a subsequent ULtransmission such that one or more other UL transmissions from one ormore other stations reach the AP at the same time as the subsequent ULtransmission.

Certain embodiments of the present disclosure provide a method forranging a device in a wireless communications system. The methodgenerally includes receiving a UL signal based on an OFDM frame having aplurality of OFDM symbols, performing correlation to detect one or morecode sequences from the plurality of OFDM symbols, determining timingadjustment information based on the correlation, and transmitting thetiming adjustment information.

Certain embodiments of the present disclosure provide a computer-programproduct for ranging a device in a wireless communications system. Thecomputer-program product typically includes a computer-readable mediumhaving instructions stored thereon, the instructions being executable byone or more processors. The instructions generally include instructionsfor receiving a UL signal based on an OFDM frame having a plurality ofOFDM symbols, instructions for performing correlation to detect one ormore code sequences from the plurality of OFDM symbols, instructions fordetermining timing adjustment information based on the correlation, andinstructions for transmitting the timing adjustment information.

Certain embodiments of the present disclosure provide an apparatus forwireless communications. The apparatus generally includes means forreceiving a UL signal based on an OFDM frame having a plurality of OFDMsymbols, means for performing correlation to detect one or more codesequences from the plurality of OFDM symbols, means for determiningtiming adjustment information based on the correlation, and means fortransmitting the timing adjustment information.

Certain embodiments of the present disclosure provide an AP for wirelesscommunications. The access point generally includes a receiverconfigured to receive a UL signal based on an OFDM frame having aplurality of OFDM symbols, logic for performing correlation to detectone or more code sequences from the plurality of OFDM symbols, logic fordetermining timing adjustment information based on the correlation, anda transmitter configured to transmit the timing adjustment information.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description,briefly summarized above, may be had by reference to embodiments, someof which are illustrated in the appended drawings. It is to be noted,however, that the appended drawings illustrate only certain typicalembodiments of this disclosure and are therefore not to be consideredlimiting of its scope, for the description may admit to other equallyeffective embodiments.

FIG. 1 illustrates a spatial division multiple access (SDMA)multiple-input multiple-output (MIMO) wireless system, in accordancewith certain embodiments of the present disclosure.

FIG. 2 illustrates a block diagram of an access point (AP) and two userterminals, in accordance with certain embodiments of the presentdisclosure.

FIG. 3 illustrates various components that may be utilized in a wirelessdevice, in accordance with certain embodiments of the presentdisclosure.

FIG. 4 is a flow diagram of example operations for conveying informationin a wireless communications system using code-division multiple access(CDMA) from the perspective of a user terminal, in accordance withcertain embodiments of the present disclosure.

FIG. 4A is a block diagram of means corresponding to the exampleoperations of FIG. 4 for conveying information in a wirelesscommunications system using CDMA from the perspective of a userterminal, in accordance with certain embodiments of the presentdisclosure.

FIG. 5 illustrates converting CDMA information to orthogonalfrequency-division multiplexing (OFDM) symbols, in accordance withcertain embodiments of the present disclosure.

FIG. 6 is a block diagram of an architecture for conveying channelquality indicator (CQI) and resource requests using CDMA, in accordancewith certain embodiments of the present disclosure.

FIG. 7 is a flow diagram of example operations for interpretinginformation sent using CDMA in a wireless communications system from theperspective of an access point (AP), in accordance with certainembodiments of the present disclosure.

FIG. 7A is a block diagram of means corresponding to the exampleoperations of FIG. 7 for interpreting information sent using CDMA in awireless communications system from the perspective of an AP, inaccordance with certain embodiments of the present disclosure.

FIG. 8 illustrates sending and receiving a Walsh code sequence via OFDM,in accordance with certain embodiments of the present disclosure.

FIG. 9 illustrates a metric matrix, in accordance with certainembodiments of the present disclosure.

FIG. 10 illustrates calculating a metric matrix, thresholding thematrix, and determining the delay between a user terminal and an AP in awireless communications system based on the threshold matrix, inaccordance with certain embodiments of the present disclosure.

FIG. 11 is a flow diagram of example operations for ranging in awireless communications system based on sending information using CDMAfrom the perspective of a user terminal, in accordance with certainembodiments of the present disclosure.

FIG. 11A is a block diagram of means corresponding to the exampleoperations of FIG. 11 for ranging in a wireless communications systembased on sending information using CDMA from the perspective of a userterminal, in accordance with certain embodiments of the presentdisclosure.

FIG. 12 is a flow diagram of example operations for ranging in awireless communications system based on information sent using CDMA fromthe perspective of an AP, in accordance with certain embodiments of thepresent disclosure.

FIG. 12A is a block diagram of means corresponding to the exampleoperations of FIG. 12 for ranging in a wireless communications systembased on information sent using CDMA from the perspective of an AP, inaccordance with certain embodiments of the present disclosure.

FIG. 13 illustrates a graph of correlation metrics at different Walshindices with a peak indicative of the delay between a user terminal andan AP, in accordance with certain embodiments of the present disclosure.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure provide techniques andapparatus for using code-division multiple access (CDMA) via orthogonalfrequency-division multiplexing (OFDM) to convey information from userterminals to an access point (AP) in a wireless local area network(WLAN), for example, in an effort to acquire timing information from alarge number of user terminals in a short duration. Useful for rangingand power control, this CDMA-based scheme may be scalable to a greatnumber of user terminals and may be very robust to multiple frequencyoffsets of different user terminals.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. Also as used herein, the term“legacy stations” generally refers to wireless network nodes thatsupport 802.11 n or earlier versions of the IEEE 802.11 standard.

The multi-antenna transmission techniques described herein may be usedin combination with various wireless technologies such as Code DivisionMultiple Access (CDMA), Orthogonal Frequency Division Multiplexing(OFDM), Time Division Multiple Access (TDMA), and so on. Multiple userterminals can concurrently transmit/receive data via different (1)orthogonal code channels for CDMA, (2) time slots for TDMA, or (3)sub-bands for OFDM. A CDMA system may implement IS-2000, IS-95, IS-856,Wideband-CDMA (W-CDMA), or some other standards. An OFDM system mayimplement IEEE 802.11 or some other standards. A TDMA system mayimplement GSM or some other standards. These various standards are knownin the art.

An Example MIMO System

FIG. 1 shows a multiple-access MIMO system 100 with access points anduser terminals. For simplicity, only one access point 110 is shown inFIG. 1. An access point (AP) is generally a fixed station thatcommunicates with the user terminals and may also be referred to as abase station or some other terminology. A user terminal may be fixed ormobile and may also be referred to as a mobile station, a station (STA),a client, a wireless device, or some other terminology. A user terminalmay be a wireless device, such as a cellular phone, a personal digitalassistant (PDA), a handheld device, a wireless modem, a laptop computer,a personal computer, etc.

Access point 110 may communicate with one or more user terminals 120 atany given moment on the downlink and uplink. The downlink (i.e., forwardlink) is the communication link from the access point to the userterminals, and the uplink (i.e., reverse link) is the communication linkfrom the user terminals to the access point. A user terminal may alsocommunicate peer-to-peer with another user terminal. A system controller130 couples to and provides coordination and control for the accesspoints.

While portions of the following disclosure will describe user terminals120 capable of communicating via spatial division multiple access(SDMA), for certain embodiments, the user terminals 120 may also includesome user terminals that do not support SDMA. Thus, for suchembodiments, an AP 110 may be configured to communicate with both SDMAand non-SDMA user terminals. This approach may conveniently allow olderversions of user terminals (“legacy” stations) to remain deployed in anenterprise, extending their useful lifetime, while allowing newer SDMAuser terminals to be introduced as deemed appropriate.

System 100 employs multiple transmit and multiple receive antennas fordata transmission on the downlink and uplink. Access point 110 isequipped with a number N_(ap) of antennas and represents themultiple-input (MI) for downlink transmissions and the multiple-output(MO) for uplink transmissions. A set N_(u) of selected user terminals120 collectively represents the multiple-output for downlinktransmissions and the multiple-input for uplink transmissions. For pureSDMA, it is desired to have N_(ap)≧N_(u)≧1 if the data symbol streamsfor the N_(u) user terminals are not multiplexed in code, frequency, ortime by some means. N_(u) may be greater than N_(ap) if the data symbolstreams can be multiplexed using different code channels with CDMA,disjoint sets of sub-bands with OFDM, and so on. Each selected userterminal transmits user-specific data to and/or receives user-specificdata from the access point. In general, each selected user terminal maybe equipped with one or multiple antennas (i.e., N_(ul)≧1). The N_(u)selected user terminals can have the same or different number ofantennas.

MIMO system 100 may be a time division duplex (TDD) system or afrequency division duplex (FDD) system. For a TDD system, the downlinkand uplink share the same frequency band. For an FDD system, thedownlink and uplink use different frequency bands. MIMO system 100 mayalso utilize a single carrier or multiple carriers for transmission.Each user terminal may be equipped with a single antenna (e.g., in orderto keep costs down) or multiple antennas (e.g., where the additionalcost can be supported).

FIG. 2 shows a block diagram of access point 110 and two user terminals120 m and 120 x in MIMO system 100. Access point 110 is equipped withN_(ap) antennas 224 a through 224 ap. User terminal 120 m is equippedwith N_(ut,m) antennas 252 ma through 252 mu, and user terminal 120 x isequipped with N_(ut,x) antennas 252 xa through 252 xu. Access point 110is a transmitting entity for the downlink and a receiving entity for theuplink. Each user terminal 120 is a transmitting entity for the uplinkand a receiving entity for the downlink. As used herein, a “transmittingentity” is an independently operated apparatus or device capable oftransmitting data via a wireless channel, and a “receiving entity” is anindependently operated apparatus or device capable of receiving data viaa wireless channel. In the following description, the subscript “dn”denotes the downlink, the subscript “up” denotes the uplink, N_(up) userterminals are selected for simultaneous transmission on the uplink,N_(dn) user terminals are selected for simultaneous transmission on thedownlink, N_(up) may or may not be equal to N_(dn), and N_(up) andN_(dn) may be static values or can change for each scheduling interval.The beam-steering or some other spatial processing technique may be usedat the access point and user terminal.

On the uplink, at each user terminal 120 selected for uplinktransmission, a TX data processor 288 receives traffic data from a datasource 286 and control data from a controller 280. TX data processor 288processes (e.g., encodes, interleaves, and modulates) the traffic data{d_(up,m)} for the user terminal based on the coding and modulationschemes associated with the rate selected for the user terminal andprovides a data symbol stream {s_(up,m)}. A TX spatial processor 290performs spatial processing on the data symbol stream {s_(up,m)} andprovides N_(ut,m) transmit symbol streams for the N_(ut,m) antennas.Each transmitter unit (TMTR) 254 receives and processes (e.g., convertsto analog, amplifies, filters, and frequency upconverts) a respectivetransmit symbol stream to generate an uplink signal. N_(ut,m)transmitter units 254 provide N_(ut,m) uplink signals for transmissionfrom N_(ut,m) antennas 252 to the access point 110.

A number N_(up) of user terminals may be scheduled for simultaneoustransmission on the uplink. Each of these user terminals performsspatial processing on its data symbol stream and transmits its set oftransmit symbol streams on the uplink to the access point.

At access point 110, N_(ap) antennas 224 a through 224 ap receive theuplink signals from all N_(up) user terminals transmitting on theuplink. Each antenna 224 provides a received signal to a respectivereceiver unit (RCVR) 222. Each receiver unit 222 performs processingcomplementary to that performed by transmitter unit 254 and provides areceived symbol stream. An RX spatial processor 240 performs receiverspatial processing on the N_(ap) received symbol streams from N_(ap)receiver units 222 and provides N_(up) recovered uplink data symbolstreams. The receiver spatial processing is performed in accordance withthe channel correlation matrix inversion (CCMI), minimum mean squareerror (MMSE), successive interference cancellation (SIC), or some othertechnique. Each recovered uplink data symbol stream {s_(up,m)} is anestimate of a data symbol stream {s_(up,m)} transmitted by a respectiveuser terminal. An RX data processor 242 processes (e.g., demodulates,deinterleaves, and decodes) each recovered uplink data symbol stream{s_(up,m)} in accordance with the rate used for that stream to obtaindecoded data. The decoded data for each user terminal may be provided toa data sink 244 for storage and/or a controller 230 for furtherprocessing.

On the downlink, at access point 110, a TX data processor 210 receivestraffic data from a data source 208 for N_(dn) user terminals scheduledfor downlink transmission, control data from a controller 230, andpossibly other data from a scheduler 234. The various types of data maybe sent on different transport channels. TX data processor 210 processes(e.g., encodes, interleaves, and modulates) the traffic data for eachuser terminal based on the rate selected for that user terminal. TX dataprocessor 210 provides N_(dn) downlink data symbol streams for theN_(dn) user terminals. A TX spatial processor 220 performs spatialprocessing on the N_(dn) downlink data symbol streams, and providesN_(ap) transmit symbol streams for the N_(ap) antennas. Each transmitterunit (TMTR) 222 receives and processes a respective transmit symbolstream to generate a downlink signal. N_(ap) transmitter units 222provide N_(ap) downlink signals for transmission from N_(ap) antennas224 to the user terminals.

At each user terminal 120, N_(ut,m) antennas 252 receive the N_(ap)downlink signals from access point 110. Each receiver unit (RCVR) 254processes a received signal from an associated antenna 252 and providesa received symbol stream. An RX spatial processor 260 performs receiverspatial processing on N_(ut,m) received symbol streams from N_(ut,m)receiver units 254 and provides a recovered downlink data symbol stream{s_(dn,m)} for the user terminal. The receiver spatial processing isperformed in accordance with the CCMI, MMSE, or some other technique. AnRX data processor 270 processes (e.g., demodulates, deinterleaves, anddecodes) the recovered downlink data symbol stream to obtain decodeddata for the user terminal.

At each user terminal 120, N_(ut,m) antennas 252 receive the N_(ap)downlink signals from access point 110. Each receiver unit (RCVR) 254processes a received signal from an associated antenna 252 and providesa received symbol stream. An RX spatial processor 260 performs receiverspatial processing on N_(ut,m) received symbol streams from N_(ut,m)receiver units 254 and provides a recovered downlink data symbol stream{s_(dn,m)} for the user terminal. The receiver spatial processing isperformed in accordance with the CCMI, MMSE, or some other technique. AnRX data processor 270 processes (e.g., demodulates, deinterleaves, anddecodes) the recovered downlink data symbol stream to obtain decodeddata for the user terminal.

FIG. 3 illustrates various components that may be utilized in a wirelessdevice 302 that may be employed within the system 100. The wirelessdevice 302 is an example of a device that may be configured to implementthe various methods described herein. The wireless device 302 may be anaccess point 110 or a user terminal 120.

The wireless device 302 may include a processor 304 which controlsoperation of the wireless device 302. The processor 304 may also bereferred to as a central processing unit (CPU). Memory 306, which mayinclude both read-only memory (ROM) and random access memory (RAM),provides instructions and data to the processor 304. A portion of thememory 306 may also include non-volatile random access memory (NVRAM).The processor 304 typically performs logical and arithmetic operationsbased on program instructions stored within the memory 306. Theinstructions in the memory 306 may be executable to implement themethods described herein.

The wireless device 302 may also include a housing 308 that may includea transmitter 310 and a receiver 312 to allow transmission and receptionof data between the wireless device 302 and a remote location. Thetransmitter 310 and receiver 312 may be combined into a transceiver 314.A plurality of transmit antennas 316 may be attached to the housing 308and electrically coupled to the transceiver 314. The wireless device 302may also include (not shown) multiple transmitters, multiple receivers,and multiple transceivers.

The wireless device 302 may also include a signal detector 318 that maybe used in an effort to detect and quantify the level of signalsreceived by the transceiver 314. The signal detector 318 may detect suchsignals as total energy, energy per subcarrier per symbol, powerspectral density and other signals. The wireless device 302 may alsoinclude a digital signal processor (DSP) 320 for use in processingsignals.

The various components of the wireless device 302 may be coupledtogether by a bus system 322, which may include a power bus, a controlsignal bus, and a status signal bus in addition to a data bus.

Using CDMA to Send Uplink Signals in WLANs

In future wireless local area network (WLAN) systems, user terminals maybe required to request for resources before every uplink SDMAtransmission. Using a contention-based scheme without access point (AP)feedback at the end may be used, but this scheme is inefficient in termsof the time it takes to get a certain number of requests through. Thisis because a high number of slots may be required to ensure a low numberof collisions. A scheme with AP feedback may allow for collisions, butis still not efficient when few users request for resources.

Furthermore, multiple user terminals trying to access the networksimultaneously creates problems for a wireless communications systemand, more specifically, for an AP. For example, it may be desirable totime synchronize the multiple user terminals at the AP simultaneously.The ability to send a small number of information bits on a channel mayalso be desired. This information may be used, for example, for sendingallocation indication (AI), which carries requests for resources (REQs).As another example, the information may include channel qualityindicator (CQI) information. As yet another example, channel estimationat the AP may be performed by sending a pilot sequence.

Accordingly, what are needed are techniques and apparatus for a multipleaccess scheme which scales well with the number of users and does notincur too much overhead.

One option might be to use orthogonal frequency-division multiple access(OFDMA). With OFDMA, each user terminal gets a dedicated block offrequency resources, which tends to limit the number of user terminalswhich can access the system simultaneously due to diversity issues. Foran example of 64 user terminals, assigning one tone to each userterminal yields very poor diversity. Also, the AP may need tocommunicate the frequency blocks each user terminal may use fortransmission. Furthermore, if the block size is fixed, a user terminalnot being there is just a wasted resource.

A second option may be to use CDMA for transmitting the uplinkinformation. With CDMA, the user terminals may get full degrees offreedom at the expense of a signal-to-interference-plus-noise ratio(SINR) degradation. However, processing gain may compensate for any SINRdegradation. Furthermore, one user terminal not transmitting benefitsall other users by bringing the interference level down. This may leadto a graceful degradation with the number of user terminals. In otherwords, a CDMA-based scheme can support a large number of user terminalsin a relatively low number of slots without suffering from having asmall number of user terminals. Moreover, no scheduling information isneeded with a CDMA solution.

Therefore, the basic mechanism behind certain embodiments of the presentdisclosure may involve a user terminal employing CDMA to transmit arequest/ranging message to the AP in a period called the CDMA controlopportunity. The AP may respond by telling the user terminal to advanceor retard timing by a certain number of samples and may also sendfeedback for resource allocation. The information sent may include anysuitable uplink message, such as a resource request (REQ), a channelquality indicator (CQI), a beamforming metric, or other metric variantspertaining to channel quality. For some embodiments, the information maybe a known pilot sequence. Different values of CQI or REQ may beconveyed by employing different indices (e.g., Walsh indices), whereeach index refers to a different CQI or REQ value.

Although embodiments of the present disclosure refer to Walsh sequencesand indices, any suitable N-bit code sequence and corresponding indicesmay be used. For example, certain embodiments may derive the N-bit codesequence from an IEEE 802.11b waveform. As another example, certainembodiments may use a Barker sequence or a modified Barker sequence.

Furthermore, since multiple user terminals may be transmittinginformation simultaneously, the user terminals may be differentiatedthrough quadrature phase-shift keying (QPSK) scrambling. In other words,after a Walsh sequence is selected, the sequence may be multiplied witha complex QPSK scrambling sequence. The scrambling seed used to generatethe scrambling sequence may be station ID dependent (i.e., dependent onthe identification number of the user terminal).

FIG. 4 is a flow diagram of example operations 400, from the perspectiveof a user terminal, for conveying information in a wirelesscommunications system using CDMA. The operations 400 may begin, at 410,by selecting a Walsh index indicative of information to be conveyed inan uplink (UL) signal. The information to be conveyed may be anysuitable UL information, such as a channel quality indicator (CQI) or aresource request (REQ). The UL signal may comprise one or more signalstransmitted from a user terminal 120 to an AP 110 (in an infrastructuremode of deployment) or from one STA to one or more other STAs (in apeer-to-peer application mode or ad hoc deployment).

At 420, an N-bit Walsh code sequence corresponding to the Walsh indexmay be divided into M Walsh code subsequences. At 430, the Msubsequences may be transmitted as a plurality of orthogonalfrequency-division multiplexing (OFDM) symbols in an OFDM frame.

FIG. 5 illustrates converting CDMA information to OFDM symbols, as agraphical example of the operations 400 of FIG. 4. For example, a Walshcode sequence 500 having a length of N=1024 bits and comprising anysuitable sequence of digital 0's and 1's may be split into M=64subsequences (Y_(k), 0≦k≦63) at 510, each sequence having 16 samples520. At 530, a fast Fourier transform (FFT) or a discrete Fouriertransform (DFT) may be applied to each of the 64 subsequences such thateach subsequence generates 16 tones at 540. The 16 tones for eachsubsequence may correspond to an OFDM symbol, such that the 64subsequences may produce 64 OFDM symbols (Z_(k), 0≦k≦63) to betransmitted in 16 tones of a CDMA sub-band 550. In other words, the 64subsequences may be transmitted by transmission spanning 64 OFDMsymbols.

FIG. 6 is an example block diagram 600 of a proposed architecture forconveying uplink information, such as channel quality indicator (CQI)and resource requests (REQs) using CDMA, according to certainembodiments of the present disclosure. A Walsh index, representative ofthe information to be conveyed, may be input to a Walsh sequenceselection block 602 to select a Walsh sequence. The output Walshsequence may be scrambled (e.g., QPSK scrambled) by a complexmultiplication block 604 such that different user terminals usedifferent scramblings. The scrambled sequence may be input to a powermeasuring/adjustment block 606. For some embodiments, the output of thepower block 606 may be input to an interleaving block 608. Theinterleaving block 608 may interleave the output of the power block 606with a permutation sequence of 1024 elements, for example.

In any event, the processed digital signal (Y_(CQI) or Y_(REQ)) for eachmessage may be summed in a summing block 610, and the sum (Y_(CDMA)) maybe divided into subsequences at block 612. For example, block 612 maydivide the sum into 64 subsequences with a length of 16 samples. Thesubsequences (Y_(k)) may be sent to an FFT block (not shown) or adiscrete Fourier transform (DFT) block 614 to be transformed into tones,which may be transmitted as OFDM symbols (Z_(k)). As illustrated in FIG.6, the DFT block 614 may be a 16-point DFT block to transform 64subsequences of length 16 into 64 OFDM symbols, each symbol having 16tones. Although only two processing paths for UL information areportrayed in the block diagram 600, any suitable number of processingpaths may be included in the architecture such that a correspondingnumber of processed Walsh indices may be summed by the summing block 610to form sum Y_(CDMA).

Once the tones have been transmitted by the user terminal as an OFDMtransmission, an AP may try to interpret information in the received ULsignal. FIG. 7 is a flow diagram of example operations 700, from theperspective of the AP, for interpreting information sent using CDMA in awireless communications system. The operations 700 may begin, at 710, byreceiving an uplink signal based on an OFDM frame having a plurality ofOFDM symbols.

At 720, the AP may extract M subsequences for a Walsh code sequence fromthe plurality of OFDM symbols. The AP may correlate the M subsequenceswith reference Walsh code sequences at 730. At 740, the AP may determinethe Walsh code sequence form the correlation such that a Walsh indexcorresponding to the Walsh code sequence indicates conveyed information.For certain embodiments, if the conveyed information was a resourcerequest (REQ), the AP may transmit an acknowledgment of the REQ forreception by the user terminal. For other embodiments, if the conveyedinformation was a CQI, the AP may transmit a downlink (DL) signal basedon the CQI.

For some embodiments, the AP may determine the Walsh code sequence bycomparing a threshold to correlation metrics and considering referenceWalsh code sequences corresponding to the metrics above the threshold asthe received Walsh code sequence(s). This method is described in greaterdetail below.

FIG. 8 illustrates sending and receiving a Walsh sequence 800 via OFDM.The Walsh sequence 800 may be divided into subsequences and convertedinto tones (i.e., OFDM subcarriers) as described above, where eachconverted subsequence 810 is on a different OFDM symbol 805. After theuser terminal transmits the OFDM symbols 805 in an OFDM frame via thewireless channel, the AP may receive the OFDM symbols 815 after acertain delay.

At the AP, each subsequence 820 may be extracted from the received OFDMsymbol 815. A correlation between each subsequence 820 and a referenceWalsh code subsequence 830 may be performed. Furthermore, the referenceWalsh code subsequence 830 may be cyclically shifted by a delay d, and acorrelation between each subsequence 820 and the cyclically shiftedreference Walsh code subsequence may be determined for all shiftedsequences or up to a predetermined number of cyclical shifts. The finalcorrelation metric may be the sum of all correlations performed on thesubsequences 820 for a given reference Walsh code sequence used (and allreceiving antennas) at a particular shift.

FIG. 9 illustrates an example metric matrix 900 having a plurality ofcells 902. The metric matrix 900 may be arranged according to thereference Walsh code sequence used (or more practically to the referencesequence's corresponding Walsh index 904 as shown) and to the cyclicalshift 906 of that reference sequence, in integer multiples of delay d.For some embodiments, the metric calculation for each cell 902 may be:

$M_{w,d} = \frac{\sum\limits_{a = 1}^{Nr}\; {c_{w,d,a}}^{2}}{\frac{({PN})^{2}}{2}}$

where M is the final metric, w is the Walsh index of the reference Walshcode sequence used, d is the shift index, a is the antenna receiving thedata, Nr is the number of receiving antennas, P is the power of thereceived signal, and N is the normalization of the reference sequence.The correlation c for each antenna a, Walsh index w, and shift index dmay be calculated as

$c_{w,d,a} = {\sum\limits_{u = 1}^{U}\; {s_{w,d}^{u} \cdot r_{a}^{u^{*}}}}$

where u is the OFDM symbol, U is the number of OFDM symbols, s is theshifted reference sequence, and r is the received sequence. The power Pof the received signal may be calculated as

$P = \sqrt{\sum\limits_{a,{u = 1}}^{{Nr},U}\; {r_{a}^{u} \cdot r_{a}^{u^{*}}}}$

Correlating M subsequences with reference N-bit code sequences tocalculate the correlation metrics for each cell 902 may include: (a)selecting one of the reference N-bit code sequences; (b) dividing theselected reference N-bit code sequence into M reference subsequences;(c) cyclically shifting each of the M reference subsequences by aninteger multiple of delay d; (d) calculating a correlation value of eachof the M subsequences with the shifted M reference subsequences; (e)summing the correlation values from (d) to determine a metric for theselected reference N-bit code sequence at the current integer multipleof delay d; (f) cyclically shifting each of the M reference subsequencesby a different integer multiple of delay d; (g) repeating steps (d)-(f)until a plurality of metrics for the selected reference N-bit codesequence has been determined for a predetermined number of cyclicalshifts; (h) selecting a different one of the reference N-bit codesequences; and (i) repeating steps (b)-(h) until metrics have beendetermined for all of the reference N-bit code sequences and thepredetermined number of cyclical shifts.

FIG. 10 illustrates a portion of an example metric matrix 1000 once themetric calculations have been performed for the cells 902 of the metricmatrix 900. The metric matrix 1000 may be thresholded to yield athresholded metric matrix 1010. Thresholding may be used to remove theeffects of noise, interference, and/or frequency offset. In thethresholded metric matrix 1010, all metric values below a certainthreshold have been replaced with zeros. Starting with the examplemetric matrix 1000, all metric values below a 9 were replaced withzeros, leaving only five columns with remaining metric values greaterthan or equal to 9 in the thresholded metric matrix 1010. The columnswith metric values greater than or equal to the threshold may beconsidered as candidate Walsh indices corresponding to informationcontained in the message. As described above, this information mayinclude a resource request (REQ) or a CQI, for example. In thethresholded matrix 1010, the five columns with metric values greaterthan or equal to 9 may indicate five different Walsh indices.

The CDMA-based scheme described above may offer a number of advantageswhen compared to conventional techniques for transmitting UL signalsfrom a large number of user terminals. First, efficiency with respect tothe number of slots used may be increased. In addition, this scheme maybe scalable to a high number of user terminals and may have a soft limitas to the number of users that can be supported. Furthermore, thisCDMA-based scheme may offer greater tolerance to frequency offsets.

The choice of a CDMA sequence of length N=512 seems to be a goodtradeoff between detection performance and amount of information sent. Areliable detection and ranging of up 25 user terminals per 5 MHz band ispossible. With the current scheme, up to ±64 chips of delay may bedetected. Performance may suffer as the delay to be detected increases.

False alarm probability highly depends on the frequency offset, at highsignal-to-interference-plus-noise ratio (SINR). Frequency offset may adda deterministic modulation to the sent symbols in the time domain. Thisdeterministic component may change the cross correlation properties ofthe received sequences. In other words, sequences that were originallyorthogonal to each other may no longer be orthogonal after transmissiondue to frequency offset, thereby leading to increased false detection.In the presence of high noise levels, the detection threshold may beincreased in an effort to prevent false detection of non-nullcross-correlations arising from frequency offset.

Interleaving may also be used to reduce the false alarm probability inthe presence of frequency offset. Therefore, for some embodiments, theuser terminal may interleave the Walsh sequence as illustrated in theinterleaving block 608 of FIG. 6. Interleaving may help to eliminatecross-correlations due to frequency offset, thereby reducing the falsealarm probability without affecting the detection probability. However,interleaving may entail increased memory requirements since all the OFDMsymbols have to be received before the sequence can be de-interleaved.Furthermore, de-interleaving may require more processing time after thelast OFDM symbol is received.

Ranging Using CDMA

In future WLAN systems, uplink SDMA may rely heavily on the timesynchronization of the user terminal signals arriving at the AP. Anotheradvantage offered by the CDMA-based scheme described above is theability to perform ranging at an AP and get timing information of alarge number of user terminals in a very short duration.

FIG. 11 is a flow diagram of example operations 1100, from theperspective of a user terminal, for ranging in a wireless communicationssystem based on sending information using CDMA. The operations 1100 maybegin, at 1110, by transmitting a plurality of OFDM symbols representinga Walsh code sequence in an OFDM frame, as described above. As describedabove, the plurality of OFDM symbols may be transmitted (as a UL signal)from a user terminal 120 to an AP 110 (in an infrastructure mode ofdeployment) or from one STA to one or more other STAs (in a peer-to-peerapplication mode or ad hoc deployment). In other words, ranging may beperformed between an AP 110 and a user terminal 120 or between a firstuser terminal and a second user terminal. The operations 1100 belowfocus on embodiments where ranging is performed between an AP 110 and auser terminal 120, although the reader will understand that the AP 110may be replaced by a second user terminal (e.g., a STA) in otherembodiments.

At 1120, the user terminal may receive timing information from the AP.This timing information may be based on when the plurality of OFDMsymbols was received by the AP. The timing information may include apropagation delay (or delay information based on said propagation delay)between the user terminal and the AP. At 1130, the user terminal may usethe timing information to control a start time of a subsequent ULtransmission. In this manner, with multiple user terminals transmittinginformation according to their respective timing information based ondifferent delays between the user terminals and the AP, the informationfrom the multiple user terminals may be received at the APsimultaneously.

Again, although embodiments of the present disclosure refer to Walshsequences and indices, any suitable N-bit code sequence andcorresponding indices may be used. For example, certain embodiments mayderive the N-bit code sequence from an IEEE 802.11b waveform. As anotherexample, certain embodiments may use a Barker sequence or a modifiedBarker sequence.

FIG. 12 is a flow diagram of example operations 1200, from theperspective of an AP, for ranging in a wireless communications systembased on information sent using CDMA. The operations 1200 may begin, at1210, by receiving an uplink signal based on an OFDM frame having aplurality of OFDM symbols. At 1220, the AP may perform correlation todetect one or more Walsh code sequences from the plurality of OFDMsymbols, as described above.

At 1230, the AP may determine timing adjustment information based on thecorrelation, which is described in greater detail below. At 1240, the APmay transmit the timing adjustment information to the AP. The operations1200 may be performed for multiple user terminals, each having differenttiming adjustment information.

Referring back to FIG. 10, the thresholded metric matrix 1010 maydetermine which Walsh indices were likely included in the transmittedmessage, as described above. The cell 1022 with the highest metric maybe selected as the cell representing the most accurate estimate of thepropagation delay between the user terminal and the AP as shown inmatrix 1020. The delay d of the shift index corresponding to the cell1022 may represent the propagation delay. This delay d may be used tocalculate the timing adjustment information, which may be transmitted bythe AP to the user terminal.

FIG. 13 illustrates a graph 1300 of correlation metrics at 512 differentWalsh indices after thresholding an example metric matrix and selectingone shift index to represent the propagation delay between the AP and aparticular user terminal, from which the timing adjustment informationintended for that user terminal may be calculated. Moreover, the peak1310 may indicate that a Walsh code is present, and the Walsh indexcorresponding to the peak 1310 may represent the received information.

The various operations of methods described above may be performed byvarious hardware and/or software component(s) and/or module(s)corresponding to means-plus-function blocks illustrated in the figures.Generally, where there are methods illustrated in figures havingcorresponding counterpart means-plus-function figures, the operationblocks correspond to means-plus-function blocks with similar numbering.For example, blocks 410-430 illustrated in FIG. 4 correspond tomeans-plus-function blocks 410A-430A illustrated in FIG. 4A, and blocks1210-1240 illustrated in FIG. 12 correspond to means-plus-functionblocks 1210A-1240A illustrated in FIG. 12A.

As used herein, the term “determining” encompasses a wide variety ofactions. For example, “determining” may include calculating, computing,processing, deriving, investigating, looking up (e.g., looking up in atable, a database or another data structure), ascertaining and the like.Also, “determining” may include receiving (e.g., receiving information),accessing (e.g., accessing data in a memory) and the like. Also,“determining” may include resolving, selecting, choosing, establishingand the like.

Information and signals may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals and the like that may be referencedthroughout the above description may be represented by voltages,currents, electromagnetic waves, magnetic fields or particles, opticalfields or particles or any combination thereof.

The various illustrative logical blocks, modules and circuits describedin connection with the present disclosure may be implemented orperformed with a general purpose processor, a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array signal (FPGA) or other programmable logic device(PLD), discrete gate or transistor logic, discrete hardware componentsor any combination thereof designed to perform the functions describedherein. A general purpose processor may be a microprocessor, but in thealternative, the processor may be any commercially available processor,controller, microcontroller or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thepresent disclosure may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in any form of storage medium that is knownin the art. Some examples of storage media that may be used includerandom access memory (RAM), read only memory (ROM), flash memory, EPROMmemory, EEPROM memory, registers, a hard disk, a removable disk, aCD-ROM and so forth. A software module may comprise a singleinstruction, or many instructions, and may be distributed over severaldifferent code segments, among different programs, and across multiplestorage media. A storage medium may be coupled to a processor such thatthe processor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor.

The methods disclosed herein comprise one or more steps or actions forachieving the described method. The method steps and/or actions may beinterchanged with one another without departing from the scope of theclaims. In other words, unless a specific order of steps or actions isspecified, the order and/or use of specific steps and/or actions may bemodified without departing from the scope of the claims.

The functions described may be implemented in hardware, software,firmware, or any combination thereof. If implemented in software, thefunctions may be stored as one or more instructions on acomputer-readable medium. A storage media may be any available mediathat can be accessed by a computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code in the form of instructions or datastructures and that can be accessed by a computer. Disk and disc, asused herein, include compact disc (CD), laser disc, optical disc,digital versatile disc (DVD), floppy disk, and Blu-ray® disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers.

Software or instructions may also be transmitted over a transmissionmedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition oftransmission medium.

Further, it should be appreciated that modules and/or other appropriatemeans for performing the methods and techniques described herein can bedownloaded and/or otherwise obtained by a user terminal and/or basestation as applicable. For example, such a device can be coupled to aserver to facilitate the transfer of means for performing the methodsdescribed herein. Alternatively, various methods described herein can beprovided via storage means (e.g., RAM, ROM, a physical storage mediumsuch as a compact disc (CD) or floppy disk, etc.), such that a userterminal and/or base station can obtain the various methods uponcoupling or providing the storage means to the device. Moreover, anyother suitable technique for providing the methods and techniquesdescribed herein to a device can be utilized.

It is to be understood that the claims are not limited to the preciseconfiguration and components illustrated above. Various modifications,changes and variations may be made in the arrangement, operation anddetails of the methods and apparatus described above without departingfrom the scope of the claims.

1. A method for ranging a device in a wireless communications system,comprising: transmitting a plurality of orthogonal frequency-divisionmultiplexed (OFDM) symbols representing a code sequence in an OFDMframe; receiving timing information from an access point (AP) or a userterminal, the timing information based on when the plurality of OFDMsymbols was received by the AP or the user terminal; and using thetiming information to control a start time of a subsequent uplink (UL)transmission.
 2. The method of claim 1, wherein the code sequencecomprises a Walsh sequence.
 3. The method of claim 1, wherein the codesequence is derived from an IEEE 802.11b Barker sequence.
 4. The methodof claim 1, wherein the code sequence comprises a code-division multipleaccess (CDMA) sequence.
 5. The method of claim 1, wherein the wirelesscommunications system comprises a Wireless Local Area Network (WLAN)system according to an IEEE 802.11 family of standards.
 6. The method ofclaim 1, further comprising: selecting an index indicative ofinformation to be conveyed in a UL signal; and dividing the codesequence corresponding to the index into subsequences, whereintransmitting the plurality of OFDM symbols comprises transmitting thesubsequences as the plurality of OFDM symbols.
 7. The method of claim 6,further comprising quadrature phase-shift keying (QPSK) scrambling thecode sequence corresponding to the index to indicate one of a pluralityof user terminals simultaneously transmitting UL signals, whereindividing the code sequence comprises dividing the QPSK-scrambled codesequence into the subsequences.
 8. The method of claim 6, wherein theinformation comprises at least one of a resource request, a channelquality indicator (CQI), a known pilot sequence, a beamforming metric,and other metric variants pertaining to channel quality.
 9. The methodof claim 6, wherein the code sequence comprises a 1024-bit code sequenceand dividing the code sequence comprises dividing the 1024-bit codesequence into 64 subsequences, each subsequence having 16 samples. 10.The method of claim 9, wherein transmitting the 64 subsequencescomprises transmission spanning 64 OFDM symbols, wherein 16 subcarriersare used in each of the OFDM symbols.
 11. The method of claim 1, whereinusing the timing information comprises delaying the start time of thesubsequent UL transmission according to the timing information.
 12. Themethod of claim 1, wherein the UL transmission comprises one or moresignals transmitted from the device to the user terminal in apeer-to-peer application mode or from the device to the AP.
 13. Acomputer-program product for ranging a device in a wirelesscommunications system, comprising a computer-readable medium havinginstructions stored thereon, the instructions being executable by one ormore processors and the instructions comprising: instructions fortransmitting a plurality of orthogonal frequency-division multiplexed(OFDM) symbols representing a code sequence in an OFDM frame;instructions for receiving timing information from an access point (AP)or a user terminal, the timing information based on when the pluralityof OFDM symbols was received by the AP or the user terminal; andinstructions for using the timing information to control a start time ofa subsequent uplink (UL) transmission.
 14. The computer-program productof claim 13, wherein the code sequence comprises a Walsh sequence. 15.The computer-program product of claim 13, wherein the code sequence isderived from an IEEE 802.11b Barker sequence.
 16. The computer-programproduct of claim 13, wherein the code sequence comprises a code-divisionmultiple access (CDMA) sequence.
 17. The computer-program product ofclaim 13, wherein the wireless communications system comprises aWireless Local Area Network (WLAN) system according to an IEEE 802.11family of standards.
 18. The computer-program product of claim 13,further comprising: instructions for selecting an index indicative ofinformation to be conveyed in a UL signal; and instructions for dividingthe code sequence corresponding to the index into subsequences, whereinthe instructions for transmitting the plurality of OFDM symbols compriseinstructions for transmitting the subsequences as the plurality of OFDMsymbols.
 19. The computer-program product of claim 18, furthercomprising instructions for quadrature phase-shift keying (QPSK)scrambling the code sequence corresponding to the index to indicate oneof a plurality of user terminals simultaneously transmitting UL signals,wherein the instructions for dividing the code sequence compriseinstructions for dividing the QPSK-scrambled code sequence into thesubsequences.
 20. The computer-program product of claim 18, wherein theinformation comprises at least one of a resource request, a channelquality indicator (CQI), a known pilot sequence, a beamforming metric,and other metric variants pertaining to channel quality.
 21. Thecomputer-program product of claim 18, wherein the code sequencecomprises a 1024-bit code sequence and the instructions for dividing thecode sequence comprise instructions for dividing the 1024-bit codesequence into 64 subsequences, each subsequence having 16 samples. 22.The computer-program product of claim 21, wherein the instructions fortransmitting the 64 subsequences comprise instructions for transmissionspanning 64 OFDM symbols, wherein 16 subcarriers are used in each of theOFDM symbols.
 23. The computer-program product of claim 13, wherein theinstructions for using the timing information comprise instructions fordelaying the start time of the subsequent UL transmission according tothe timing information.
 24. The computer-program product of claim 13,wherein the UL transmission comprises one or more signals transmittedfrom the device to the user terminal in a peer-to-peer application modeor from the device to the AP.
 25. An apparatus for wirelesscommunications, comprising: means for transmitting a plurality oforthogonal frequency-division multiplexed (OFDM) symbols representing acode sequence in an OFDM frame; means for receiving timing informationfrom an access point (AP) or a user terminal, the timing informationbased on when the plurality of OFDM symbols was received by the AP orthe user terminal; and means for using the timing information to controla start time of a subsequent uplink (UL) transmission.
 26. The apparatusof claim 25, wherein the code sequence comprises a Walsh sequence. 27.The apparatus of claim 25, wherein the code sequence is derived from anIEEE 802.11b Barker sequence.
 28. The apparatus of claim 25, wherein thecode sequence comprises a code-division multiple access (CDMA) sequence.29. The apparatus of claim 25, wherein the apparatus is part of aWireless Local Area Network (WLAN) system according to an IEEE 802.11family of standards.
 30. The apparatus of claim 25, further comprising:means for selecting an index indicative of information to be conveyed ina UL signal; and means for dividing the code sequence corresponding tothe index into subsequences, wherein the means for transmitting theplurality of OFDM symbols is configured to transmit the subsequences asthe plurality of OFDM symbols.
 31. The apparatus of claim 30, furthercomprising means for quadrature phase-shift keying (QPSK) scrambling thecode sequence corresponding to the index to indicate one of a pluralityof user terminals simultaneously transmitting UL signals, wherein themeans for dividing the code sequence is configured to divide theQPSK-scrambled code sequence into the subsequences.
 32. The apparatus ofclaim 30, wherein the information comprises at least one of a resourcerequest, a channel quality indicator (CQI), a known pilot sequence, abeamforming metric, and other metric variants pertaining to channelquality.
 33. The apparatus of claim 30, wherein the code sequencecomprises a 1024-bit code sequence and the means for dividing the codesequence are configured to divide the 1024-bit code sequence into 64subsequences, each subsequence having 16 samples.
 34. The apparatus ofclaim 33, wherein the means for transmitting the 64 subsequences isconfigured to transmission span 64 OFDM symbols, wherein 16 subcarriersare used in each of the OFDM symbols.
 35. The apparatus of claim 25,wherein the means for using the timing information is configured todelay the start time of the subsequent UL transmission according to thetiming information.
 36. The apparatus of claim 25, wherein the ULtransmission comprises one or more signals transmitted from theapparatus to the user terminal in a peer-to-peer application mode orfrom the apparatus to the AP.
 37. A mobile device, comprising: atransmitter configured to transmit a plurality of orthogonalfrequency-division multiplexed (OFDM) symbols representing a codesequence in an OFDM frame; a receiver configured to receive timinginformation from an access point (AP) or another mobile device, thetiming information based on when the plurality of OFDM symbols wasreceived by the AP or the other mobile device; and logic for using thetiming information to control a start time of a subsequent uplink (UL)transmission.
 38. The mobile device of claim 37, wherein the codesequence comprises a Walsh sequence.
 39. The mobile device of claim 37,wherein the code sequence is derived from an IEEE 802.11b Barkersequence.
 40. The mobile device of claim 37, wherein the code sequencecomprises a code-division multiple access (CDMA) sequence.
 41. Themobile device of claim 37, wherein the mobile device is part of aWireless Local Area Network (WLAN) system according to an IEEE 802.11family of standards.
 42. The mobile device of claim 37, furthercomprising: logic for selecting an index indicative of information to beconveyed in a UL signal; and logic for dividing the code sequencecorresponding to the index into subsequences, wherein the transmitter isconfigured to transmit the subsequences as the plurality of OFDMsymbols.
 43. The mobile device of claim 42, further comprising logic forquadrature phase-shift keying (QPSK) scrambling the code sequencecorresponding to the index to indicate one of a plurality of userterminals simultaneously transmitting UL signals, wherein the logic fordividing the code sequence is configured to divide the QPSK-scrambledcode sequence into the subsequences.
 44. The mobile device of claim 42,wherein the information comprises at least one of a resource request, achannel quality indicator (CQI), a known pilot sequence, a beamformingmetric, and other metric variants pertaining to channel quality.
 45. Themobile device of claim 42, wherein the code sequence comprises a1024-bit code sequence and the logic for dividing the code sequence isconfigured to divide the 1024-bit code sequence into 64 subsequences,each subsequence having 16 samples.
 46. The mobile device of claim 45,wherein the transmitter is configured to transmission span 64 OFDMsymbols, wherein 16 subcarriers are used in each of the OFDM symbols.47. The mobile device of claim 37, wherein the logic for using thetiming information is configured to delay the start time of thesubsequent UL transmission according to the timing information.
 48. Themobile device of claim 37, wherein the UL transmission comprises one ormore signals transmitted from the mobile device to the other mobiledevice in a peer-to-peer application mode or from the mobile device tothe AP.
 49. A method for ranging a device in a wireless communicationssystem, comprising: receiving an uplink (UL) signal based on anorthogonal frequency-division multiplexed (OFDM) frame having aplurality of OFDM symbols; performing correlation to detect one or morecode sequences from the plurality of OFDM symbols; determining timingadjustment information based on the correlation; and transmitting thetiming adjustment information.
 50. The method of claim 49, wherein theone or more code sequences comprise a Walsh sequence.
 51. The method ofclaim 49, wherein the one or more code sequences are derived from anIEEE 802.11b Barker sequence.
 52. The method of claim 49, wherein theone or more code sequences comprise a code-division multiple access(CDMA) sequence.
 53. The method of claim 49, wherein determining thetiming adjustment information comprises selecting a delay correspondingto the highest correlation value from the correlation.
 54. The method ofclaim 49, wherein performing the correlation comprises: extracting codesubsequences from the plurality of OFDM symbols; correlating thesubsequences with reference code sequences; and determining the one ormore code sequences from the correlation.
 55. The method of claim 54,wherein correlating the subsequences with the reference code sequencescomprises calculating metrics, one for each of the reference codesequences, by summing correlation values between each of thesubsequences and a corresponding reference code sequence.
 56. The methodof claim 55, wherein determining the one or more code sequencescomprises: comparing a threshold to the metrics; and considering, asreceived code sequences, the reference code sequences corresponding tothe metrics above the threshold.
 57. The method of claim 54, whereincorrelating the subsequences with the reference code sequencescomprises: (a) selecting one of the reference code sequences; (b)dividing the selected reference code sequence into referencesubsequences; (c) cyclically shifting each of the reference subsequencesby an integer multiple of delay d; (d) calculating a correlation valueof each of the subsequences with the shifted reference subsequences; (e)summing the correlation values from (d) to determine a metric for theselected reference code sequence at the current integer multiple of thedelay d; (f) cyclically shifting each of the reference subsequences by adifferent integer multiple of delay d; (g) repeating steps (d)-(f) untilmetrics for the selected reference code sequence have been determinedfor a predetermined number of cyclical shifts; (h) selecting a differentone of the reference code sequences; and (i) repeating steps (b)-(h)until the metrics have been determined for all of the reference codesequences and the predetermined number of cyclical shifts.
 58. Themethod of claim 57, wherein determining the one or more code sequencescomprises: comparing a threshold to the metrics; and considering, asreceived code sequences, the reference code sequences corresponding tothe metrics above the threshold.
 59. The method of claim 57, whereindetermining the timing adjustment information comprises: selecting oneof the cyclically shifted reference code sequences with the highestmetric out of the metrics; determining the integer multiple of the delayd used for the selected cyclically shifted reference code sequence; andcalculating the timing adjustment information based on the integermultiple of the delay d used.
 60. The method of claim 49, wherein anindex corresponding to at least one of the code sequences indicates ULinformation.
 61. The method of claim 60, wherein the UL informationcomprises at least one of a resource request (REQ), a channel qualityindicator (CQI), a known pilot sequence, a beamforming metric, and othermetric variants pertaining to channel quality.
 62. A computer-programproduct for ranging a device in a wireless communications system,comprising a computer-readable medium having instructions storedthereon, the instructions being executable by one or more processors andthe instructions comprising: instructions for receiving an uplink (UL)signal based on an orthogonal frequency-division multiplexed (OFDM)frame having a plurality of OFDM symbols; instructions for performingcorrelation to detect one or more code sequences from the plurality ofOFDM symbols; instructions for determining timing adjustment informationbased on the correlation; and instructions for transmitting the timingadjustment information.
 63. The computer-program product of claim 62,wherein the one or more code sequences comprise a Walsh sequence. 64.The computer-program product of claim 62, wherein the one or more codesequences are derived from an IEEE 802.11b Barker sequence.
 65. Thecomputer-program product of claim 62, wherein the one or more codesequences comprise a code-division multiple access (CDMA) sequence. 66.The computer-program product of claim 62, wherein the instructions fordetermining the timing adjustment information comprise instructions forselecting a delay corresponding to the highest correlation value fromthe correlation.
 67. The computer-program product of claim 62, whereinthe instructions for performing the correlation comprise: instructionsfor extracting code subsequences from the plurality of OFDM symbols;instructions for correlating the subsequences with reference codesequences; and instructions for determining the one or more codesequences from the correlation.
 68. The computer-program product ofclaim 67, wherein the instructions for correlating the subsequences withthe reference code sequences comprise instructions for calculatingmetrics, one for each of the reference code sequences, by summingcorrelation values between each of the subsequences and a correspondingreference code sequence.
 69. The computer-program product of claim 68,wherein the instructions for determining the one or more code sequencescomprise: instructions for comparing a threshold to the metrics; andinstructions for considering, as received code sequences, the referencecode sequences corresponding to the metrics above the threshold.
 70. Thecomputer-program product of claim 67, wherein the instructions forperforming the correlation comprise: (a) instructions for selecting oneof the reference code sequences; (b) instructions for dividing theselected reference code sequence into reference subsequences; (c)instructions for cyclically shifting each of the reference subsequencesby an integer multiple of delay d; (d) instructions for calculating acorrelation value of each of the subsequences with the shifted referencesubsequences; (e) instructions for summing the correlation values from(d) to determine a metric for the selected reference code sequence atthe current integer multiple of the delay d; (f) instructions forcyclically shifting each of the reference subsequences by a differentinteger multiple of delay d; (g) instructions for repeating steps(d)-(f) until metrics for the selected reference code sequence have beendetermined for a predetermined number of cyclical shifts; (h)instructions for selecting a different one of the reference codesequences; and (i) instructions for repeating steps (b)-(h) until themetrics have been determined for all of the reference code sequences andthe predetermined number of cyclical shifts.
 71. The computer-programproduct of claim 70, wherein the instructions for determining the one ormore code sequences comprise: instructions for comparing a threshold tothe metrics; and instructions for considering, as received codesequences, the reference code sequences corresponding to the metricsabove the threshold.
 72. The computer-program product of claim 70,wherein the instructions for determining the timing adjustmentinformation comprise: instructions for selecting one of the cyclicallyshifted reference code sequences with the highest metric out of themetrics; instructions for determining the integer multiple of the delayd used for the selected cyclically shifted reference code sequence; andinstructions for calculating the timing adjustment information based onthe integer multiple of the delay d used.
 73. The computer-programproduct of claim 62, wherein an index corresponding to at least one ofthe code sequences indicates UL information.
 74. The computer-programproduct of claim 73, wherein the UL information comprises at least oneof a resource request (REQ), a channel quality indicator (CQI), a knownpilot sequence, a beamforming metric, and other metric variantspertaining to channel quality.
 75. An apparatus for wirelesscommunications, comprising: means for receiving an uplink (UL) signalbased on an orthogonal frequency-division multiplexed (OFDM) framehaving a plurality of OFDM symbols; means for performing correlation todetect one or more code sequences from the plurality of OFDM symbols;means for determining timing adjustment information based on thecorrelation; and means for transmitting the timing adjustmentinformation.
 76. The apparatus of claim 75, wherein the one or more codesequences comprise a Walsh sequence.
 77. The apparatus of claim 75,wherein the one or more code sequences are derived from an IEEE 802.11bBarker sequence.
 78. The apparatus of claim 75, wherein the one or morecode sequences comprise a code-division multiple access (CDMA) sequence.79. The apparatus of claim 75, wherein the means for determining thetiming adjustment information is configured to select a delaycorresponding to the highest correlation value from the correlation. 80.The apparatus of claim 75, wherein the means for performing thecorrelation is configured to extract code subsequences from theplurality of OFDM symbols, correlate the subsequences with referencecode sequences, and determine the one or more code sequences from thecorrelation.
 81. The apparatus of claim 80, wherein the means forperforming the correlation is configured to correlate the subsequenceswith the reference code sequences by calculating metrics, one for eachof the reference code sequences, by summing correlation values betweeneach of the subsequences and a corresponding reference code sequence.82. The apparatus of claim 81, wherein the means for performing thecorrelation is configured to determine the one or more code sequences bycomparing a threshold to the metrics and considering, as received codesequences, the reference code sequences corresponding to the metricsabove the threshold.
 83. The apparatus of claim 80, wherein the meansfor performing the correlation is configured to: (a) select one of thereference code sequences; (b) divide the selected reference codesequence into reference subsequences; (c) cyclically shift each of thereference subsequences by an integer multiple of a delay d; (d)calculate a correlation value of each of the subsequences with theshifted reference subsequences; (e) sum the correlation values from (d)to determine a metric for the selected reference code sequence at thecurrent integer multiple of the delay d; (f) cyclically shift each ofthe reference subsequences by a different integer multiple of the delayd; (g) repeat steps (d)-(f) until metrics for the selected referencecode sequence have been determined for a predetermined number ofcyclical shifts; (h) select a different one of the reference codesequences; and (i) repeat steps (b)-(h) until the metrics have beendetermined for all of the reference code sequences and the predeterminednumber of cyclical shifts.
 84. The apparatus of claim 83, wherein themeans for performing the correlation is configured to determine the oneor more code sequences by comparing a threshold to the metrics andconsidering, as received code sequences, the reference code sequencescorresponding to the metrics above the threshold.
 85. The apparatus ofclaim 83, wherein the means for determining the timing adjustmentinformation is configured to select one of the cyclically shiftedreference code sequences with the highest metric out of the metrics,determine the integer multiple of the delay d used for the selectedcyclically shifted reference code sequence, and calculate the timingadjustment information based on the integer multiple of the delay dused.
 86. The apparatus of claim 75, wherein an index corresponding toat least one of the code sequences indicates UL information.
 87. Theapparatus of claim 86, wherein the UL information comprises at least oneof a resource request (REQ), a channel quality indicator (CQI), a knownpilot sequence, a beamforming metric, and other metric variantspertaining to channel quality.
 88. An access point (AP) for wirelesscommunication, comprising: a receiver configured to receive an uplink(UL) signal based on an orthogonal frequency-division multiplexed (OFDM)frame having a plurality of OFDM symbols; logic for performingcorrelation to detect one or more code sequences from the plurality ofOFDM symbols; logic for determining timing adjustment information basedon the correlation; and a transmitter configured to transmit the timingadjustment information.
 89. The access point of claim 88, wherein theone or more code sequences comprise a Walsh sequence.
 90. The accesspoint of claim 88, wherein the one or more code sequences are derivedfrom an IEEE 802.11b Barker sequence.
 91. The access point of claim 88,wherein the one or more code sequences comprise a code-division multipleaccess (CDMA) sequence.
 92. The access point of claim 88, wherein thelogic for determining the timing adjustment information is configured toselect a delay corresponding to the highest correlation value from thecorrelation.
 93. The access point of claim 88, wherein the logic forperforming the correlation is configured to extract code subsequencesfrom the plurality of OFDM symbols, correlate the subsequences withreference code sequences, and determine the one or more code sequencesfrom the correlation.
 94. The access point of claim 93, wherein thelogic for performing the correlation is configured to correlate thesubsequences with the reference code sequences by calculating metrics,one for each of the reference code sequences, by summing correlationvalues between each of the subsequences and a corresponding referencecode sequence.
 95. The access point of claim 94, wherein the logic forperforming the correlation is configured to determine the one or morecode sequences by comparing a threshold to the metrics and considering,as received code sequences, the reference code sequences correspondingto the metrics above the threshold.
 96. The access point of claim 93,wherein the logic for performing the correlation is configured to: (a)select one of the reference code sequences; (b) divide the selectedreference code sequence into reference subsequences; (c) cyclicallyshift each of the reference subsequences by an integer multiple of delayd; (d) calculate a correlation value of each of the subsequences withthe shifted reference subsequences; (e) sum the correlation values from(d) to determine a metric for the selected reference code sequence atthe current integer multiple of the delay d; (f) cyclically shift eachof the reference subsequences by a different integer multiple of delayd; (g) repeat steps (d)-(f) until metrics for the selected referencecode sequence have been determined for a predetermined number ofcyclical shifts; (h) select a different one of the reference codesequences; and (i) repeat steps (b)-(h) until the metrics have beendetermined for all of the reference code sequences and the predeterminednumber of cyclical shifts.
 97. The access point of claim 96, wherein thelogic for performing the correlation is configured to determine the oneor more code sequences by comparing a threshold to the metrics andconsidering, as received code sequences, the reference code sequencescorresponding to the metrics above the threshold.
 98. The access pointof claim 96, wherein the logic for determining the timing adjustmentinformation is configured to select one of the cyclically shiftedreference code sequences with the highest metric out of the metrics,determine the integer multiple of the delay d used for the selectedcyclically shifted reference code sequence, and calculate the timingadjustment information based on the integer multiple of the delay dused.
 99. The access point of claim 88, wherein an index correspondingto at least one of the code sequences indicates UL information.
 100. Theaccess point of claim 99, wherein the UL information comprises at leastone of a resource request (REQ), a channel quality indicator (CQI), aknown pilot sequence, a beamforming metric, and other metric variantspertaining to channel quality.
 101. A method for ranging multipledevices in a wireless communications system, comprising: transmitting aplurality of orthogonal frequency-division multiplexed (OFDM) symbols inan OFDM frame; receiving timing information from an access point (AP),the timing information based on when the plurality of OFDM symbols wasreceived by the AP; and using the timing information to control a starttime of a subsequent uplink (UL) transmission such that one or moreother UL transmissions from one or more other stations reach the AP atthe same time as the subsequent UL transmission.
 102. A wirelesscommunications system, comprising: a user terminal configured totransmit a plurality of orthogonal frequency-division multiplexed (OFDM)symbols representing a code sequence in an OFDM frame; and an accesspoint (AP) configured to receive the plurality of symbols in the OFDMframe, perform correlation to detect the code sequence from theplurality of OFDM symbols, determine timing information based on thecorrelation and when the plurality of OFDM symbols was received by theAP, and transmit the timing information such that the user terminal mayuse the timing information to control a start time of a subsequentuplink (UL) transmission.
 103. The system of claim 102, wherein the codesequence comprises a Walsh sequence.
 104. The system of claim 102,wherein the code sequence is derived from an IEEE 802.11b Barkersequence.
 105. The system of claim 102, wherein the code sequencecomprises a code-division multiple access (CDMA) sequence.
 106. Thesystem of claim 102, wherein the wireless communications systemcomprises a Wireless Local Area Network (WLAN) system according to anIEEE 802.11 family of standards.
 107. A wireless communications system,comprising: a plurality of user terminals, wherein each user terminal isconfigured to transmit a plurality of orthogonal frequency-divisionmultiplexed (OFDM) symbols representing a code sequence in an OFDM framesuch that a plurality of OFDM frames are simultaneously transmitted bythe plurality of user terminals; and an access point (AP) configured toreceive the plurality of OFDM frames and, for each received OFDM framein the plurality of OFDM frames, configured to perform correlation todetect the code sequence from the plurality of OFDM symbols in the OFDMframe, determine timing information based on the correlation and on whenthe plurality of OFDM frames was received by the AP, and transmit thetiming information such that the plurality of user terminals may use thetiming information for each of the plurality of user terminals tocontrol start times of subsequent uplink (UL) transmissions to besimultaneously received by the AP.
 108. The system of claim 107, whereinthe plurality of user terminals are differentiated through quadraturephase-shift keying (QPSK) scrambling.